The estimated day of ovulation and the fertile window around it, worked out from your cycle length rather than assumed to be day 14.
These are estimates from a calendar rule, not medical advice, and not a contraceptive method.
The day bleeding started, not the day it ended.
From the first day of one period to the first day of the next.
Estimated fertile window
Estimated ovulation
Next period expected
The next three cycles
What this does not tell you
The familiar "ovulation happens on day 14" holds only for a 28-day cycle, and most cycles are not 28 days. The stable part of a cycle is the second half: from ovulation to the next period is roughly 14 days, with little variation between people. What stretches and shrinks is the first half. So in a 35-day cycle the estimate lands near day 21, and in a 24-day cycle near day 10 — counting backwards from the next expected period, not forwards from the last one.
The fertile window is not centred on ovulation, and the asymmetry is the point. Sperm survive up to about five days in the reproductive tract; an egg survives 12 to 24 hours. That makes the window the five days before ovulation plus the day itself — six days, almost all of them beforehand. A window drawn symmetrically around the estimated day would be half wrong, and wrong on the side that carries the information.
None of this is a contraceptive method. Calendar-based fertility tracking is the least reliable form of birth control there is: with typical use, about 24 in 100 people become pregnant within a year, against fewer than 1 in 100 with modern methods. The dates here are estimates from a calendar rule, not a measurement — ovulation shifts between cycles even in people whose cycles are regular, and very short or very long cycles are more often anovulatory, meaning no egg is released at all.
By counting backwards. The next period is expected one cycle length after the first day of the last one, and ovulation is estimated 14 days before that — the luteal phase, which is the stable half. On a 28-day cycle that lands on day 14; on a 35-day cycle it lands near day 21.
Because sperm outlive the egg. Sperm can remain viable in the reproductive tract for up to about five days, while an egg survives 12 to 24 hours after release. The six-day window that follows from those two figures is the one described in the fertility literature, notably Wilcox, Weinberg and Baird in the New England Journal of Medicine in 1995.
No. Calendar-based tracking is the least reliable form of contraception measured: about 24 pregnancies per 100 people per year with typical use. The estimate here is a calendar rule with no knowledge of your body, and it cannot see the cycle-to-cycle shift that makes the method fail.
A calendar estimate assumes the next cycle resembles the one you entered, so the less regular the cycles are, the less the dates mean. This calculator accepts cycles from 21 to 45 days and shows no date outside that range, because there the rule no longer describes what is happening. Ovulation tests and basal body temperature measure the cycle rather than assume it.
No. It runs entirely in your browser, in a few lines of JavaScript. This site is static files, so there is no server to send anything to and nothing is stored.
Body mass index from your height and weight, with the range it falls in.
Estimated daily energy use from the Mifflin-St Jeor equation, with its margin.
Estimated due date, gestational age and trimester, adjusted for your cycle.
Add, subtract, multiply and divide fractions — exactly, never rounded.
Solve a proportion — with the inverse case the usual formula gets wrong.
What is X% of Y — the everyday percentage, worked out step by step.